In U.S. Pat. No. 4,863,260, apparatus is disclosed for quantitatively assessing and displaying corneal topography. The dioptric refraction exhibited over the corneal surface is shown in a color-coded display that employs a distinctive color for each fraction of a diopter present. The topographic display is obtained by illuminating the cornea with a pattern of structured light. The structured light is provided by a series of concentric rings spaced along a translucent, illuminated plastic cylinder. The apex of the patient's cornea is positioned at point at adjacent the open end of the illuminated cylinder at which two laser beams intersect. In calibrating the apparatus a series of slightly different diameter perfect spheres are positioned at the point determined by the intersection of the laser beams. The spacing of the illuminated rings in the plastic cylinder is such that the structured light pattern appearing on the perfect spheres is a series of equally spaced rings. A two-dimensional video image of the ring pattern appearing is acquired. The extent to which the structured light pattern appearing on a similarly positioned human cornea departs from the pattern exhibited by a perfect sphere is used to determine the corneal topography. The video image of the ring pattern is radially scanned and the coordinates of points which appear to lie on the rings are determined. From the two-dimensional coordinates of such points the processing apparatus constructs a model of the three-dimensional corneal surface.
Although the prior art color display provides much useful topographic information and has gained wide acceptance, the assessment of the patient's vision ultimately depends on whether the desired dioptric powers are so located as to focus an image through the pupil. Accordingly, it would be useful to provide the exact location of the pupil on the topographic display. However, the prior art apparatus, which analyzed the ring pattern appearing on the corneal surface to provide topographic data, yielded no information about the pupil since the pupil is not on the corneal surface and had nothing to do with the topography of the corneal surface being measured.